How to keep particles moving around inside an invisible box
I think "brownian motion" is probably what you're looking for.



The setup for this example includes two cubes: one as the emitter, and one (slightly larger) as a collision object, so that the particles don't wander too far.  These particles are emitted from the volume and I gave them a very slight starting velocity from the normals, but most of the motion comes from the Brownian force  under "Physics".  It is important to give the particles a small amount of drag and/or damp, otherwise the Brownian motion will keep adding up and the particles will increase in velocity over time.